The problem seams to be the kind of collection you are using.

A SortedCollection holds elements sorted by a given criteria (In the form of a block).��

When you add elements to an instance of SortedCollection, the rule is execute with the new object added, and the contained object to determinate the object position.
Now, the at: put: message in this kind of collections (SortedCollection is a specific class of OrderedCollection), is used to put an object instead of another.

But in a SortedCollection, if you replace an object by other, then you are breaking the sort of the collection.

That's why SortedCollection overrides at:put: as self shouldNotImplement, which raises an exception indicating that you send a message that should not be implemented by that object (Although for me, this reflects a bad design in the hierarchy of Collection)

So the possible solutions are:
-If what you want is sort the elements by a giving criteria, use add: to add the objects to the collection.
-If you want to sort the collection by the order in which you added the elements, use OrderedCollection instead. You should use add: for this to, as at: put: is used to replace an object by another.
-If you have a fixed number of elements to add, you should use an Array, you should set the object of the array using at: put:

Hope that helps.

And, if I misunderstood you problem, please forgive me.

Cheers
--
Alan Rodas Bonjour